drm/i915: Ensure the HW is powered when accessing the CRC HW block

The assumption when adding the intel_display_power_is_enabled() checks
was that if it returns success the power can't be turned off afterwards
during the HW access, which is guaranteed by modeset locks. This isn't
always true, so make sure we hold a dedicated reference for the time of
the access.

While at it also add the missing reference around the HW access in
i915_interrupt_info().

v2:
- update the commit message mentioning that this also fixes the
  HW access in the interrupt info debugfs entry (Daniel)

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455296121-4742-9-git-send-email-imre.deak@intel.com